Generally, materials which are suitable as high temperature stable permanent resists are comprised of high molecular weight polymers. Such films must be capable of having a preselected pattern formed therein (i.e., by lithographic patterning/etching) with either exposed or unexposed regions being soluble in a solvent while the other of the exposed or unexposed region being relatively insoluble in the solvent. In addition, such films including interlayer dielectric films must have inherently good mechanical properties. Good mechanical properties are characterized by high modulus, high elongation at break and low tendency to exhibit solvent induced (environmental) crazing and/or cracking. High molecular weight long chain polymers usually exhibit better mechanical properties then their low molecular weight analogs. However, high molecular weight unfavorably influences other important characteristics of good resist materials, such as solvent diffusion rate, speed of dissolution, contrast, resolution, and line profile.
In general, materials with good lithographic properties exhibit high sensitivity to radiation such as optical light (ultraviolet and/or visible), electron beam (e-beam), x-ray, or ion-beam radiation and have a significant solubility differential between exposed and unexposed regions. High contrast is identified by the existence of a sharp solubility difference observed for a very narrow dose range. A film having a high contrast minimizes the deleterious effects caused by the scattering of radiation in the resist film. In addition, good lithographic materials exhibit a minimum amount of swelling and are capable of high resolution.
For optimum lithographic performance low molecular weight polymers are preferred over their high molecular weight analogs because the former offer advantages with respect to their solubility differential and their speed of dissolution. Low molecular weight materials exhibit higher contrast and subsequently higher resolution than high molecular weight materials.
Oligomers, however, exhibit poorer mechanical properties than their high molecular weight analogs because the shorter chains do not allow viscoelasticity dissipation of significant amounts of energy, thus causing the films to become brittle. For this reason, oligomers generally are not used as permanent resist films for semiconductor devices, even though they offer superior permanent resist lithographic properties than higher molecular weight materials, as discussed above.
The practitioner who desires to apply a film to a substrate is also aware of the need for the film to have good planarization and low viscosity. It is frequently difficult to find a material exhibiting both good planarization and a low viscosity in solution because high planarization requires the solution to have a high solids content which even for oligomers tends to increase the viscosity. It is known in the art that the higher the solids content of a solution, the higher the solution viscosity. Therefore, a balance must be reached for good planarization and low viscosity.
Generally, the % planarization of a coating is described by: ##EQU1## where h.sub.1 and h.sub.2 are height values represented in schematic FIG. 5. Where h.sub.2 is zero, the planarization is 100%. A film with 100% planarization is most desirable. If 100% planarization is reached after spinning the film, shrinkage accompanying the subsequent drying process will lead to a smaller decrease in the percent of planarization when the solvent content is lower because the solids content is higher.
Typically, high molecular weight polymers have a planarization value of approximately 30%. An increase in the planarization value by increasing the solids content is generally not a viable option because a solution containing more than 20% solids of a high molecular weight polymer, such as polyamic acid, a polyimide precursor, will be too viscous for the known procedures of depositing the film such as spinning or spraying the solution. Therefore, solutions of high molecular weight polymers have limitations with respect to the acheivable planarization and viscosity values.
Solutions consisting of oligomers, however, have much lower solution viscosities. The solids content of said solutions can, therefore, be increased substantially without reaching the limits of the processability. High planarization values are attainable with oligomers. This advantage, however, cannot usually be exploited due to the poor mechanical properties exhibited by oligomer films.
To avoid the difficulties arising from the high molecular weight of linear macromolecules, and still obtain good mechanical properties, resist technology has relied on using cross-linking systems in which mixtures of relatively low molecular weight, reactive molecules are combined with radiation-sensitive reagents which start the cross-linking reaction upon exposure. Cross-linking processes like these eventually provide very high molecular weight polymers and with acceptable mechanical properties.
Despite the success of resists based on cross-linking systems, two major areas are in need of improvement. First, cross-linked systems tend to be highly brittle due to the high cross-link densities reached with the low molecular weight starting molecules. Secondly, most of the materials available in this area are thermally stable only to intermediate temperatures rarely exceeding 250.degree. C.
Linear macromolecules usually show much better impact resistance, i.e., they are much less brittle. However, as pointed out above, the shortcomings of high molecular weight polymers for resist applications substantially limit their use.
In general, there is no teaching in the art which provides for a method of using a high molecular weight polymer to prepare a film and which simultaneously avoids compromising the best planarization, mechanical and lithographic properties.